A woman was beheaded by an attacker with a knife who also killed two other people at a church in the French city of Nice on Thursday, police said, in an incident the city’s mayor described as terrorism.

Mayor Christian Estrosi later said that the attacker had yelled “Allahu Akbar” (God is greatest) repeatedly, even after he was detained by police.

Estrosi said on Twitter that the knife attack had happened in or near the city's Notre Dame church and that police had detained the attacker.

Police said three people were confirmed to have died in the attack and several were injured. The French anti-terrorist prosecutor's department said it had been asked to investigate.

One of the people killed inside the church was believed to be the church warden, Estrosi said, adding that a woman had tried to escape from inside the church and had fled into a bar opposite the 19th-century neo-Gothic building.

“The suspected knife attacker was shot by police while being detained, he is on his way to hospital, he is alive,” Estrosi told reporters.

“Enough is enough,” Estrosi said. “It's time now for France to exonerate itself from the laws of peace in order to definitively wipe out Islamo-fascism from our territory.”

The mayor said the victims had been killed in a “horrible way”.

“The methods match, without doubt, those used against the brave teacher in Conflans Sainte Honorine, Samuel Paty,” he said, referring to a French teacher beheaded earlier this month in an attack in a suburb of Paris.

A police source said a woman was decapitated. French politician Marine Le Pen also spoke of a decapitation having occurred in the attack.

Reuters journalists at the scene said police armed with automatic weapons had put up a security cordon around the church, which is on Nice’s Jean Medecin avenue, the city’s main shopping thoroughfare. Ambulances and fire service vehicles were also at the scene.

Security alert raised to highest level

Following the attack in Nice, France's Prime Minister Jean Castex said that the security alert for French territory had been raised to the highest level.

Castex also told the French National Assembly that the government's response to the attack would be firm and implacable.

Condemnation and solidarity

French President Emmanuel Macron is due to visit Nice, Estrosi said.

In Paris, lawmakers in the National Assembly observed a minute's silence in solidarity with the victims.

A representative of the French Council for the Muslim Faith strongly condemned the attack.

“As a sign of mourning and solidarity with the victims and their loved ones, I call on all Muslims in France to cancel all the celebrations of the holiday of Mawlid.” The holiday of 12th of Rabiul Awal is the birthday of Holy Prophet Mohammad (PBUH), which is being celebrated in France on Thursday.

Condemnations poured in from the global community as well, as Britain and Germany expressed solidarity and European Commission President Ursula von der Leyen termed the attack as "odious and brutal".

Von der Leyen further said that the whole of Europe stood in solidarity with France and remained united and determined “in the face of barbarism and fanaticism”.

Pakistan, in a statement issued by the Foreign Office, condemned the attack, saying that "there is no justification for such acts of violence, in particular in places of worship". It also extended condolences to the families of the victims.

Turkey also denounced the “savage” knife attack, offering its “solidarity” despite a running diplomatic spat with Paris.

“We strongly condemn the attack committed today inside the Notre-Dame church in Nice,” a foreign ministry statement said while offering condolences to the victims' relatives.

The Vatican said that terrorism and violence were never acceptable.

“Today's attack sowed death in a place of love and consolation, the house of the Lord,” spokesman Matteo Bruni said in a statement.

“It is a moment of pain in a time of confusion. Terrorism and violence can never be accepted.”

He said Pope Francis had been informed and was praying for the victims. The pope hoped the French people would "respond in a united way to evil with good".

Russia in its comment on the attack said it was unacceptable to kill people, but added that it was also wrong to insult the feelings of religious believers.

Man shot dead after threatening passerby

In a separate incident, French police shot dead a man in Montfavet, near the city of Avignon in southern France, after he had earlier threatened passersby with a handgun, police said, confirming media reports.

According to French radio station Europe 1, the man had shouted “Allahu Akbar” (God is greatest).

Teacher's murder

The attack comes while France is still reeling from the beheading earlier this month of French middle school teacher Paty in Paris by a man of Chechen origin.

The attacker had said he wanted to punish Paty for showing pupils sketches of the Prophet Muhammad (peace be upon him) in a civics lesson.

It was not immediately clear what the motive was for the Nice attack, or if there was any connection to the blasphemous sketches.

Since Paty's killing, French officials — backed by many citizens — have re-asserted the right to display the sketches, and the images have been widely displayed at marches in solidarity with the killed teacher.

That has prompted an outpouring of anger in parts of the Muslim world, with some governments accusing French leader Emmanuel Macron of pursuing an anti-Islam agenda.

On Sunday, Prime Minister Imran Khan denounced Macron’s remarks on blasphemous caricatures, calling them “encouragement of Islamophobia”.

He was referring to comments made by President Macron in which he criticised Islamists and vowed not to “give up cartoons” depicting Prophet Muhammad (peace be upon him).

“By attacking Islam, clearly without having any understanding of it, President Macron has attacked and hurt the sentiments of millions of Muslims in Europe and across the world,” said Prime Minister Imran, adding that “the last thing the world wants or needs is further polarisation”.

He also asked social media giant Facebook to place a ban on Islamophobia and hate against Islam just as it had placed on the Holocaust.

Additional input from Naveed Siddiqui and AFP.